The British sent 30,000 troops into the battle, including several thousand Hessians, the feared German mercenaries who earned bounties by killing Americans.
The Americans are said to have sent 10,000 troops to Brooklyn. Reno believes the number was closer to 5,000.
Washington's young farmers and tradesmen were no match for the well-organized, better-equipped, battle-hardened and much larger enemy. The Battle of Brooklyn quickly turned into a rout.
"It wasn't long before the British were overrunning everybody concerned," Reno said. "The soldiers from many of the other states, primarily the New England states, were just dropping their weapons and running for their lives.
"And you can really understand that in the heat of battle. There were cases that men would try to surrender and the Hessians would surround them with their bayonets in place and simply walked forward in a circle and until they killed everyone. It was just slaughter and bloodshed all over the field."
Cpl. William McMillan, a Marylander of 19 or 20 at the time of the battle, described the carnage.
"My captain was killed, first lieutenant was killed, second lieutenant shot through the hand, two sergeants was killed; one in front of me," he wrote in a letter uncovered by the researchers. "Two corporals killed. All belonged to our company."
Washington, watching from a distance, decided to withdraw his soldiers. He had Brig. Gen. William Alexander send companies from Delaware, Pennsylvania and Maryland to cover the American retreat.
Eventually, the Delawareans and Pennsylvanians joined the pullout, leaving four or five companies from Maryland — about 400 men — to attack the British at a stone farmhouse on elevated ground between the Gowanus Swamp and safety.
"The British had cannons up there and were firing on the men as they tried to run by," Reno said. "The Americans charged the house six times. Each time they retreated, they stepped over more and more bodies of their comrades."
'An incredible sacrifice'
That engagement would remain the bloodiest battle of the Revolutionary War. Of the 400 Marylanders, nearly 260 were killed or captured. But their stand would allow the rest of the Continental Army to escape, and Washington to rethink his tactics.
After Brooklyn, he would avoid open confrontation with the British in favor of quick strikes followed by equally rapid retreats. The Americans fought for five more years before Cornwallis surrendered to Washington at Yorktown in 1781.
"The Declaration of Independence was signed in ink in Philadelphia and ratified in blood in Brooklyn," one pundit wrote. The 19th century historian Thomas Field wrote that the Marylanders' stand was "an hour more precious to American liberty than any other."
Today, however, the Maryland 400 might be better known in New York than in their home state. There are memorials and plaques around Brooklyn and a commemoration each August (Adkins and Gov. Martin O'Malley have attended in recent years). The farmhouse, Old Stone House, was rebuilt as a museum and receives thousands of schoolchildren each year.
"It's one of the great stories of Brooklyn history, and it's also about what commitment to country means," said Kim Maier, the museum's executive director. "This was an incredible sacrifice. They took this upon themselves. It was a sacrifice mission."
The Old Stone House, on what is now 3rd Street in Brooklyn, flies two flags: an early version of the Stars and Stripes and, in honor of the Marylanders, the familiar black-and-gold standard of Lord Baltimore.
At the Maryland State Archives, Lourie's team has included a recent college graduate and two students.
Emily Huebner, who graduated from Goucher College last spring, was struck by the youth of Maryland's officer corps. She pointed at one list.
"Just this group, they're all the same age as when I started college. And here they're drawing up wills and preparing to die."